Boris Johnson’s over-exuberance on the sports field was in evidence again on Thursday when he flattened an opponent in what was supposed to be a casual game of street rugby in Tokyo.

Worse still for the mayor of London, the player he bulldozed to the ground as he hurtled down the pitch, ball in hand, was a 10-year-old schoolboy.

Johnson was taking part in what was billed as an informal game of street rugby with adults and children on the final day of his three-day trade mission to Japan when he took possession of the ball and raced for the line.

Standing in his way on the mini-pitch was Toki Sekiguchi, a Tokyo schoolboy, who hit the turf after Johnson opted for a shoulder barge rather than a change in direction. After both players had picked themselves up, Johnson went over to Toki and asked if he was all right, before shaking his hand. “I’m so sorry,” he told the somewhat shocked boy.

Toki said he suffered some pain but was soon up and running again. Photograph: Stefan Rousseau/PA

The pair later posed for photographs and Johnson gave Toki a 2015 rugby World Cup ball.

The boy looked unfazed as he said of his brush with the politician: “I felt a little bit of pain but it’s OK,” adding, perhaps improbably, that meeting Johnson had been “enjoyable”.

Johnson joked about the incident later during a speech to the British and American chambers of commerce about the benefits of hosting major sports events. The rugby World Cup will be held in Japan in 2019, and the following year Tokyo will host the summer Olympics.

“We have just played a game of street rugby with a bunch of kids and I accidentally flattened a 10-year-old, on TV unfortunately,” he said. “But, he bounced back, he put it behind him, the smile returned rapidly to his face.

“That is my theme tonight … the possibility that confidence can suddenly and unexpectedly return.”

He promised his hosts that the public and media criticism that usually precedes an Olympics would disappear once the Games were under way.

“For about a month Britain was crop-dusted with serotonin and the Games themselves were utterly spectacular,” he said of the 2012 Olympics.

Nor was the rugby incident the only occasion that caused Johnson mild embarrassment in Tokyo. Earlier in the day, in a speech to Japanese finance and technology experts, he attempted to play up London’s role as a global financial and technical hub by producing his debit card and boasting how he could now use it to pay for journeys on the city’s public transport system.

“I can get on the tube, I can get on a bus and just wave it in the general direction of the cashless receiver and, completely painlessly, very small amounts are deducted from my bank account,” he told the audience at the British ambassador’s residence.

His sense of wonderment vanished, though, when a member of the audience politely pointed out that Japanese commuters had been using contactless smart cards on public transport since 2001. “Really?” Johnson asked. “Doesn’t this undermine my whole raison d’être for being here?”